Magnetic detent torque in a motor may be desired in order to hold the position of the rotor when the motor is not operating (turned off). The detent torque effect is the attraction between the magnets and the stator poles when at rest (no current flowing), and is due to position dependent magnetic flux induced by a permanent magnetization. This is particularly advantageous when the motor forms part of an actuator.
European patent EP0689278 describes the idea of creating a detent torque generated by additional structures on the stator parts.
U.S. Pat. No. 6,753,632 describes how to achieve controlled detent torque by modifying the geometry of the magnet poles.
Both methods suffer from the problem that once the geometry has been fixed and the motor is produced, one cannot change the detent torque without changing the whole manufacturing process.